This invention relates generally to flowmeters of the oscillating ball type for measuring low flow rates, and more particularly to a meter of this type which is highly accurate and has long-term stability, the meter being substantially immune to changes in temperature, noise and other extraneous factors.
In recent years the need has arisen for flowmeters and flow controllers for pilot plants and plants manufacturing such materials as pharmaceuticals and rare chemicals where extremely low flow rates are encountered. Conventional flow-meters of the head meter or variable-area type are incapable of providing accurate measurement with signal transmission at extremely low flow rates.
One approach heretofore taken toward accurately measuring extremely low flow rates is that disclosed in the Spencer U.S. Pat. No. 3,662,598. In the Spencer oscillating-ball flowmeter, a ferromagnetic ball disposed within a flow tube is shifted therein in the direction of fluid flow and is repeatedly returned to its original position by actuating a magnetic return system when the ball intercepts a light beam. The transit time of the ball or its oscillatory frequency is related to flow rate and thereby serves as an indication thereof.
In a Spencer-type meter, the flow tube is maintained in a horizontal position; hence there is no gravitational component included in the vectors which determine the ball position, for only magnetic and fluid drag forces act on the ball. Thus in the absence of flow or when there is an extremely low flow rate, the ball rests on the lower surface of the glass or plastic flow tube and some degree of friction is encountered, which resistance affects the accuracy of the instrument. Any accidental small departure from a truly horizontal position may introduce unwanted gravitational forces of sizeable and random magnitude and direction with respect to the flow direction, thereby causing large zero shifts. Moreover, since a meter of the Spencer type employs a magnet whose force is always horizontal and is opposed to the flow direction, this meter is incapable of sensing less than a minimum flow imposed by friction or by small residual magnetism.
In the improved oscillating-ball flowmeter disclosed in the Head et al. U.S. Pat. No. 4,051,723, whose entire disclosure is incorporated herein by reference, the fluid to be metered is conducted upwardly through a vertical flow tube so that the ferromagnetic ball therein is subjected to the force of gravity, and in the absence of any other force tends to fall down the tube.
An electro-optical position sensor operatively associated with the tube produces a control signal when the ball intercepts a light beam. An electromagnet, when energized, produces a magnetic force attracting the ball and seeking to raise it above the light beam. A current controller is coupled to the electromagnet and is activated by the control signal to generate a magnet current for energizing the electromagnet. Because the control signal is interrupted each time the ball is lifted, this causes the magnet current to pulse and the ball to oscillate at a rate depending on the flow rate of the fluid. The frequency of this magnet current is indicated to provide a reading of flow rate.
One drawback to an oscillating-ball flowmeter of the Head et al. type is that the frequency of the output signal is inversely proportional to flow rate. This creates a problem in transmitting the flow rate reading to a process control system or a remote station, for such systems require a current whose intensity within a given range is proportional to the variable being sensed.
Another problem encountered in an oscillating-ball flowmeter of the Head et al. type is in connection with the start-up of the meter. When the power to the meter is turned off, the ferromagnetic ball returns to its rest position. At start up, when the power is first turned on, in order to lift the ball from its rest position to its regular oscillating zone in the flow tube, much more magnetic energy is required for this purpose than is called for in order to sustain ball oscillation. In some instances, the "start" trigger pulse provided in the Head et al. patent may be inadequate for start-up purposes, and the ball will remain at the rest position despite the magnetic force exerted on the ball.
Because the electro-optical position sensor which provides a control signal makes use of a light beam detected by a photosensor, noise stemming from remote light sources may disturb the operation of the sensor. Thus where these light sources are incandescent or fluorescent lamps subject to flicker at a rate determined by the power line frequency (i.e., 50 or 60 Hz), this may adversely influence the sensor. Also, because the light beam is continuously produced, the life of the electro-optical sensor may be relatively short.
Another serious problem arising with Head et al. oscillating ball flowmeters is the effect of temperature on the accuracy and reliability of the meter. Temperature changes in the magnet coil cause zero shift and also give rise to related span shifts. These temperature changes are caused by the self-heating effect of the coil current as well as environmental conditions. Moreover, the temperature of the fluid being metered may also affect the coil temperature.